The present invention concerns a hydraulically powered, failsafe spring-returned actuator for a nonrising stem valve which retains the desirable features of a linear valve operator and non-rising stem valve and overcomes the problem of previously available actuators.
Hydraulically actuated failsafe valves presently available to industry utilize a spring return piston directly connected to the valve gate. Fluid pressure applied to the piston overcomes the forces of the spring, valve body pressure acting on the area of the stem, and the frictional forces of the gate drag, to open the valve. This type of actuator has one principal advantage in that the valve body pressure acts on the stem area to close the valve and then to hold the valve closed, thus making the valve failsafe. However, in deep water well operations where ambient fluid pressure may be above expected well fluid producing pressures, fluid pressure on the stem area will reverse and tend to open the valve. Since it is conceivable that valve body fluid pressure may be below ambient pressure when the well is producing and above ambient pressure when the well is shut-in the valve body pressure effects become difficult to handle. Such a direct acting hydraulically operated valve has another potentially serious problem in that it is essentially a rising stem valve. Non-rising stem valve design has been accepted by industry as superior to the rising stem valve principally because the stem seal is a rotating seal and does not drag the stem in and out of the stem seal. Effects of surface deterioration due to corrosive well fluids or abrasive action of debris on the stem is minimized by the non-rising stem valve. Present hydraulically powered operators for subseas use are limited to either direct acting piston cylinders on rising stem valves or to hydraulic motors and gear trains on non-rising stem valves. Hydraulic motor powered operators require hydraulic power to close as well as open the valves and, consequently, do not return to a "safe position" when control power is lost.